It all began two years ago in a sixth-grade swimming class, when three boys saw "B.G." in her bathing suit and teased her about her weight.
Over the next year, according to the 11-year-old girl's mother, everything spiraled downward as she dropped to 96 pounds -- and eventually left her school.
"She had trouble walking, she couldn't carry herself anymore," said her mother, Mary V., who did not want their last name used. "Until then, her weight was proportional for her height. But after that, she chose not to eat."
The lawsuit, filed last week in the U.S. District Court for the Western District of Pennsylvania, alleges that the Pittsburgh Public Schools violated Title IX of the Education Act, which deals with gender equality. The lawsuit alleges the girl, who it refers to as B.G., was "harassed because of her sex."
The suit says the school created a "hostile educational environment" and names Frick Middle School and its principal, Wayne Walters, as defendants.
Pittsburgh Public School officials said they would "vigorously" fight the lawsuit and defended their anti-bully programs and harassment policies.
"We truly empathize with the family and certainly the student," said Lisa Fischetti, chief of staff and external affairs. "But in this case we certainly followed all of our standard procedures and protocols."
"We take bullying very seriously," she told ABCNews.com.
The lawsuit alleges that the harassment directed at B.G. was "sexual in nature," suggesting the girl was "unattractive and overweight" and that the school did nothing to prevent the bullying.
According to the CDC's National Youth Violence Prevention Resource Center, nearly 30 percent of youth in the United States -- more than 5.7 million -- are involved in bullying, either as a bully or a target, or both.
In a recent national survey of students in grades 6-10, 13 percent reported bullying others, 11 percent reported being the target of bullies and another 6 percent said that they bullied others and were bullied themselves.
Girls, in particular, are more likely than boys to report being targets of rumors and sexual comments.
Often, they are "anxious, insecure and cautious and suffer from low self-esteem, rarely defending themselves or retaliating when confronted by students who bully them," according to the CDC.
Such was the case with B.G., according to the lawsuit. She was taunted throughout the sixth grade at the magnet school, where she was a straight-A student, often making her cry, her mother said.
At first, her mother was unaware of the incidents and didn't notice the weight loss, she said.
"I thought every girl goes through ups and downs with body issues," Mary V. told ABCNews.com. "But the following year, after she went back to school, it was really dramatic."
When B.G. entered seventh grade in the fall, two more boys joined in the harassment, according to the lawsuit. While other students tried to shame the boys into stopping, the lawsuit adds, no faculty members or school administrators intervened.